The Challenges of Welding Molybdenum
Molybdenum is a refractory metal with a high melting point, high strength at high temperatures, high corrosion resistance, high thermal conductivity and low resistivity, and a low coefficient of linear expansion. These characteristics make molybdenum ideal for many applications, notably those within the defense, aerospace, energy, and nuclear energy industries.
Two properties of molybdenum that greatly influence its weldability are: 1) molybdenum is hard and brittle by nature and 2) molybdenum parts can be porous as a result of the method of fabrication.
At a certain temperature, molybdenum will break in a brittle manner, rather than a ductile manner. This phenomenon, known as ductile-brittle transition, is a common characteristic of refractory metals. Ductile-brittle transition poses a challenge during welding because the material can become brittle as it cools to room temperatures due to recrystallization and/or contamination.
Molybdenum also becomes more brittle when it absorbs even a minuscule amount of oxygen or nitrogen. Therefore, it is critical that the weld piece’s exposure to oxygen after cleaning and during the welding process is minimal. When laser welding, a non-reactive shielding or cover gas is often used to protect the part by completely covering the heat affected area with the gas and forcing out any oxygen. Thorough gas coverage can be achieved by welding the part in a laser-welding glove-box filled with pure gas. Another option would be to laser weld molybdenum in a vacuum, or use electron beam welding, which normally occurs in a vacuum.
Porosity issues in molybdenum parts are typically a result how the part was originally fabricated. Molybdenum parts are often made by utilizing powder metallurgy fabrication in order to yield a fine grain structure. However, poorly done, this process can result in a high rate of micropore defects. During welding, the high-pressure gases in the micropores expand rapidly after being released into the weld pool and deteriorate the strength of the joints.
Molybdenum and Laser Welding
Laser welding is a good option for welding molybdenum because it has a high power density, which allows for deep, narrow welds, small heat-affected zones, and high heating and cooling rates.
Recommended Lasers for Molybdenum Welding
The primary laser types that would be suitable for welding molybdenum are:
- Nd:YAG (Neodymium: Yttrium-Aluminum-Garnet)
- Fiber (generally Ytterbium doped)
- Disk (Yb:YAG ytterbium)
The choice of laser type is depends on operational costs and joint configurations and accessibility, as all options produce quality welds. However, the different characteristics of each occasionally cause some types of lasers to be preferable for certain applications.
Molybdenum parts can be laser welded by a continuous beam, as opposed to using a pulsed laser. This is because continuous-wave laser welding is more suited for deep penetration and is a better option for welding crack sensitive materials like molybdenum.
In CW laser welding, a laser beam is steadily applied. It can either be moved across a stationary work piece, or the work piece can be moved and the laser held stationary. A CW laser produces a continuous, “keyhole” style weld. Continuous wave lasers can be fed at speeds from 25 to 100 inches per minute to avoid heat deformation of the weld parts, which also results in an efficient and cost effective process.
Laser stir welding is also a possibility when dealing with materials like molybdenum. In laser stir welding, the beam is modulated in a pattern which causes a “stirring” of the weld pool. The result of the stirring action is decreased porosity and cracking, as well as additional capabilities to better shape and control the geometry of the weld pool and the weld itself.
Cover Gases for Molybdenum Welding
Cover gases are a critical piece of the laser welding process because they protect the weld metal from reacting with elements in the environment, such as oxygen, hydrogen, and nitrogen. The right cover gas helps maintain a stable and uncontaminated weld pool. Special attention should be given to assuring that the heated area of the weld part is completely covered with gas—this is especially important with molybdenum given its propensity for porosity. This demands utilizing particular fixtures to guarantee the complete flooding of the weld and surrounding area with gas. The optimal set up for this laser welding molybdenum would be to utilize a glove box flooded with a pure gas. Generally, however, cover gasses are less a design choice and more a production decision.
Following are some choices for cover gases:
- Argon is generally the first choice for molybdenum laser welding because of its relatively low cost of the gas and because it is slightly heavier than air, making it easy to direct and cover a part.
- Helium is less ideal, due to its high cost and the fact that it is significantly lighter than air and consequently difficult to direct. However, helium might be a prudent choice for some requirements as it can provide a higher temperature weld pool, which allows for deeper weld penetration.
- Argon-Helium Mixtures are often recommended as a good compromise with a mix of properties that can be useful depending on the application.
Molybdenum does not require special machining precautions prior to welding as it is a very hard, strong material. It does need to be very clean to eliminate contamination, so care must be taken to remove oxides and hydrocarbon contamination from molybdenum parts. Decontamination can be accomplished mechanically or chemically. Mechanical preparation involves using stainless steel wire brushes, grinding, filing or scraping to remove any oxides. Chemical cleaning methods utilize caustic solutions and water to immerse the components and remove contamination.
Best practice is to weld the molybdenum parts immediately after cleaning. If it isn’t possible to do so, repeating the cleaning process can be avoided by storing the parts in plastic bags back filled with argon or nitrogen gas.
Guidelines for Prepping the Joints
- Use clean cloth, such as cheese cloth, or paper towels, to clean a surface with solvents. Shop rags may be contaminated with oil residue and should therefore be avoided.
- Solvent clean parts first, then clean with a stainless steel wire brush. Cleaning with a wire brush prior to solvent cleaning may contaminate the part by forcing hydrocarbons and other residue into the metal.
- A new or recently cleaned stainless steel brush should always be used when cleaning joints for welding. Older brushes might be contaminated by oils, etc., if it has been sitting around on a work bench. It is also critical not to use brushes that have been used on other metals, to avoid cross contamination.
- If any metal surface has been etched, it should be cleaned using a stainless steel wire brush, as etching can produce residuals and by-products that may alter the chemical composition of the weld pool and weaken the weld.
- Wire brushes and cutting tools should be consistently cleaned.
- Compressed shop air should not be used to blow debris off of the joint area, as it contains moisture and oil contaminates. Instead, a bottled gas such as nitrogen or argon should be used to blow off a part, if necessary.
Fixturing and Weld Design
In order to avoid mismatch, laser welding necessitates good weld fixturing. This will allow for accurate placement of the beam and, consequently, a precise, tight joint. The most accurate and precise laser welds are achieved by utilizing a computer for beam placement and welding process.
- Butt Weld:
- Fit-up tolerance of 15% of the material thickness
- Straight and square sheared edges
- Less than 25% of the material thickness for misalignment and out-of-flatness of parts
- Lap Weld (burn-through or seam weld):
- Limited air gaps between pieces, to improve weld penetration and/or feed speed
- For round welds of molybdenum, no gap (the exception would be if inert gas coverage can be maintained over the entire weld area)
- Fillet Weld:
- Square edges and good fit-up